Gerardi M.H. (ed.) The Microbiology of Anaerobic Digesters

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Part V

Digesters

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Types of Anaerobic

Digesters

143

Anaerobic digesters are capable of treating insoluble wastes and soluble waste-

waters. Insoluble wastes such as particulate and colloidal organics are considered

to be high-strength wastes and require lengthy digestion periods for hydrolysis and

solubilization. Digester retention times of at least 10–20 days are typical for

high-strength wastes.

High-rate anaerobic digesters are used for the treatment of soluble wastewaters.

Because these wastewaters do not require hydrolysis and solubilization of wastes,

much faster rates of treatment are obtained. High-rate anaerobic digesters usually

have retention times of less than 8 hours.

High-strength wastes are usually treated in suspended growth systems, whereas

soluble wastewaters are usually treated in ﬁxed-ﬁlm systems. Several anaerobic

digester processes and conﬁgurations are available for the treatment of insoluble

wastes and soluble wastewaters (Table 23.1). Each conﬁguration impacts solids

retention time (SRT) and hydraulic retention time (HRT). Minimal HRT is desired

to reduce digester volume and capital costs. Maximal SRT is desired to achieve

process stability and minimal sludge production.

BACTERIAL GROWTH—SUSPENDED

In suspended growth systems, the bacteria are suspended in the digester through

intermittent or continuous mixing action (Figure 23.1). The mixing action distrib-

utes the bacteria or biomass throughout the digester.

Because completely mixed anaerobic digesters do not incorporate a means

for retaining and concentrating the biomass, the SRT is the same as the HRT.

The Microbiology of Anaerobic Digesters, by Michael H. Gerardi

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Feed

sludge

Digested

sludge

Figure 23.1

Completely mixed anaerobic digesters are designed for relatively long HRTs. Feed

sludge can be added to the digester on a continuous or intermittent basis. Advan-

tages and disadvantages of completely mixed suspended growth digesters are listed

in Table 23.2.

BACTERIAL GROWTH—FIXED FILM

Anaerobic ﬁxed-ﬁlm (sludge blankets) systems provide a quiescent environment for

the growth of an agglutinated mass of bacteria (Figure 23.2). Because bacterial

growth requires relatively long periods of time to develop, the media used in

144 TYPES OF ANAEROBIC DIGESTERS

TABLE 23.2 Advantages and Disadvantages of Suspended Growth Anaerobic Digesters

Advantages Suitable for the treatment of particulate, colloidal, and soluble wastes

Toxic wastes may be diluted

Uniform distribution of nutrients, pH, substrate, and temperature

Disadvantages Large digester volume required to provide necessary SRT

Treatment efﬁciency may be reduced due to loss of particulate and colloidal

wastes and bacteria in digester efﬂuent

TABLE 23.1 Types of Anaerobic Digesters

Characteristic Application

Bacterial growth system Suspended

Fixed ﬁlm

Temperature Psychrophilic

Mesophilic

Thermophilic

Conﬁguration Single-stage (phase)

Two-stage (phase)

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ﬁxed-ﬁlm systems hold the bacteria in the digester for relatively long periods and

provide for long SRTs and short HRTs.

The bacteria grow as ﬁxed ﬁlms of dendritic or “stringlike” masses on the sup-

portive media or as clumps of solids within the openings or voids of the supportive

media. Fixed-ﬁlm systems usually use gravel, plastic, and rock as the supportive

media. The openings make up approximately 50% or more of the media.

Fixed-ﬁlm systems operate as ﬂow-through processes, that is, wastewater passes

over and through a bed of ﬁxed ﬁlm of bacteria growth and through entrapped

clumps of bacterial growth (Figure 23.3). Soluble organic compounds in the waste-

water are absorbed (diffuse into) by the bacteria, whereas insoluble organic com-

pounds are adsorbed (attach) to the surface of the bacteria. The ﬂow of wastewater

through ﬁxed-ﬁlm systems may be from the bottom to the top (upﬂow) or from the

top to the bottom (downﬂow) (Figure 23.4).

Because the bacteria (solids) in ﬁxed-ﬁlm systems remain in the digester for long

SRTs, the systems allow methane-forming bacteria to acclimate to toxicants such as

ammonia, sulﬁde, and formaldehyde. Therefore, anaerobic ﬁxed-ﬁlm systems with

long SRTs and short HRTs may be used to treat industrial wastewater containing

toxicants.

Numerous ﬁxed-ﬁlm systems are available for use in the digestion of municipal

and industrial wastewaters and sludges (Table 23.3 and Figure 23.5). These systems

are capable of treating a variety of wastewaters and sludges, provide good contact

BACTERIAL GROWTH—FIXED FILM 145

Influent

Rock media

Effluent

Biogas

Figure 23.2

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146 TYPES OF ANAEROBIC DIGESTERS

Biogas

Effluent

Influent

Recycle

Figure 23.3

Batch feed Biogas

Two-stage, sludge bed filter

Effluent

Figure 23.4

TABLE 23.3 Anaerobic Fixed-Film Systems

Bafﬂed reactor

Expanded bed

Expanded microcarrier bed (MCB)

Fluidized-bed reactor

Fully packed upﬂow

Hybrid ﬂow

Modular

Rotating biological contactor

Thin-ﬁlm bioreactor

Upﬂow anaerobic sludge blanket (UASB)

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between the wastes and the bacteria, and can treat wastewaters and sludges over a

relatively large range of temperature values (4–55°C) (Table 23.4).

TEMPERATURE—PSYCHROPHILIC

Psychrophilic sludge digestion and methane production occur at a relatively low

temperature range (5–20°C). Because of less than optimal activity of the anaerobic

bacteria in the psychrophilic temperature range, digestion of sludge is limited to

small-scale operations such as Imhoff tanks, septic tanks, and sludge lagoons. The

TEMPERATURE—PSYCHROPHILIC 147

Biogas

Influent wastewater

Upflow anaerobic

sludge blanket

Biogas

Influent wastewater

Upflow anaerobic

sludge blanket and

clarifier

Biogas

Influent wastewater

Recycle

Downflow packed bed

anaerobic digester

Influent wastewater

Biogas

Recycle

Baffled anaerobic digester

Figure 23.5

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temperature of the digester contents is approximately the temperature of its sur-

rounding environment and varies from season to season. Because temperatures in

psychrophilic digesters are relatively low, the SRTs of these digesters are greater

than 100 days.

TEMPERATURE—MESOPHILIC

Mesophilic sludge digestion and methane production occur at a moderate temper-

ature range (30–35°C). Mesophilic anaerobic digestion of sludge is commonly used

at municipal and industrial wastewater treatment process and offers two practical

advantages of operation compared with psychrophilic and thermophilic anaerobic

digestion. First, there are more anaerobic mesophiles in nature than there are psy-

chrophiles and thermophiles. Second, it is less expensive to maintain mesophilic

temperatures in digesters than it is to maintain thermophilic temperatures. Most

anaerobic digesters in North America operate in the mesophilic range.

TEMPERATURE—THERMOPHILIC

Thermophilic sludge digestion and methane production occur at a high tempera-

ture (50–60°C). Thermophilic anaerobic digestion of sludge is more often used at

industrial wastewater treatment plants, where process heat or steam is available to

heat digesters to the thermophilic range.

Because of the high operating temperature of these digesters, sludge digestion

and methane production occur rapidly and signiﬁcant destruction of pathogens is

achieved. However, in addition to high operation costs, thermophilic digesters do

have some signiﬁcant microbiological concerns with respect to their use in degrad-

ing sludges. The number of thermophilic methane-forming bacteria is very limited,

the bacterial growth is slow, and the bacterial population experiences a high endoge-

nous death rate. Also, the bacteria are very sensitive to ﬂuctuations in digester

temperature.

CONFIGURATION—SINGLE STAGE

A typical single-stage digester consists of only one tank or reactor. Digester

operations consist of sludge addition and withdraw, mixing, heating, gas collecting,

and supernating. These operations are possible because of stratiﬁcation of the

148 TYPES OF ANAEROBIC DIGESTERS

TABLE 23.4 Examples of Wastewaters and Sludges Treated by Anaerobic Fixed-Film

Digesters

Airport deicing ﬂuids

Contaminated groundwater

Industrial wastewaters containing high concentrations of carbohydrates

Industrial wastewaters containing high concentrations of nitrogenous compounds

Low-strength wastewaters (<600 mg/l COD) at relatively short HRTs (<6 hours)

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digester content. Stratiﬁcation results in the following layers from top to bottom of

the digester: gas, scum, supernatant, active digester sludge, and digested sludge and

grit (Figure 23.6).

Single-stage digesters are more easily upset than two-stage digesters. This is

because of the presence of the simultaneous activities of two groups of bacteria, the

CONFIGURATION—SINGLE STAGE 149

Gas

Scum

Supernatant

Active Digester Sludge

Digested Sludge and Grit

Biogas Withdrawal

Solids Withdrawal

Inlet

Outle

Figure 23.6

Sludge feed

Biogas

Acid

production

Methane

production

Biogas

Figure 23.7

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